1. Field of the Invention
This invention relates to a steering wheel with an air bag, which has a polyurethane.
2. Description of the Related Art
Moderate hardness and resilience, and excellent wear resistance are required of a polyurethane (polyurethane covering for a metal core) formed for a ring of a steering wheel. On the other hand, high elongation at break is required of a polyurethane formed for a pad of a steering wheel with an air bag so as to prevent the polyurethane from fragmenting into small pieces and dispersing when the air bag inflates or expands. It is difficult, however, to find a balance point in one polyurethane material (material used for molding a polyurethane), which satisfies every these required performances, i.e., xe2x80x9cmoderate hardness and resiliencexe2x80x9d, xe2x80x9cexcellent wear resistancexe2x80x9d and xe2x80x9chigh elongation at breakxe2x80x9d. Actually, such a balance point has not been found up to now. It is because the realization of xe2x80x9cmoderate hardness and resiliencexe2x80x9d and xe2x80x9cexcellent wear resistancexe2x80x9d requires heightening the crosslinking density, which lowers the elongation.
Therefore, according to a conventional steering wheel with an air bag, a polyurethane for a ring and spokes and a polyurethane for a pad with an air bag were molded separately as separate units. And they were molded from a polyurethane material suitable for each performance and different from each other, respectively.
Because of this separate molding, the conventional steering wheel with an air bag had a problem that the number of molding equipments and molding steps were increased.
It is, therefore, an object of this invention to provide a steering wheel with an air bag, which makes it possible to decrease the number of molding equipments and molding steps, by integrally forming a polyurethane for a pad with an air bag, at least one spoke and a ring, which can attain a good balance among performances, i.e., xe2x80x9cmoderate hardness and resiliencexe2x80x9d, xe2x80x9chigh elongation at breakxe2x80x9d, and more preferably, xe2x80x9cexcellent wear resistancexe2x80x9d so as these performances to be satisfactorily met.
To solve the above-described problem, the following measures (1) to (6) were taken.
(1) A pad with an air bag, at least one spoke and a ring were integrally formed of a polyurethane, which had 50 to 70 of Shore A type hardness (at a temperature of 25xc2x0 C.) and 150% or more of elongation at break (at a temperature of 25xc2x0 C.).
(2) A pad with an air bag, at least one spoke and a ring were integrally formed of a polyurethane, which had 50 to 70 of Shore A type hardness (at a temperature of 25xc2x0 C.), 150% or more of elongation at break (at a temperature of 25xc2x0 C.) and class 4 or more (class 4 or class 5) of wear resistance (evaluation of classes will be described later).
(3) A pad with an air bag, at least one spoke and a ring were integrally formed of a polyurethane, which was molded from a polyurethane material comprising a diphenylmethane diisocyanate (MDI) prepolymer and a polyol component, said MDI prepolymer being denaturated with a long chain glycol having an average molecular weight of 2000 or more. The reason why the MDI prepolymer is denaturated with a long chain glycol having an average molecular weight of 2000 or more, in other words, the reason why the MDI prepolymer is synthesized by reaction of a long chain glycol having an average molecular weight of 2000 or more and MDI, is to attain good balance between hardness and resilience, and elongation at break without ruining straightness of the isocyanate. The polyol component is not limited to a specific one, and a polyether polyol or a polymer polyol is preferably used.
(4) A pad with an air bag, at least one spoke and a ring were integrally formed of a polyurethane, which was molded from a polyurethane material comprising an MDI prepolymer and a polyol component, said MDI prepolymer being a mixture of a prepolymer having two benzene nucleuses and a prepolymer having three benzene nucleuses with the ratio of the prepolymer having three benzene nucleuses to the total being 7% by weight or less. The reason why the ratio of the prepolymer having three benzene nucleuses is restricted to 7% by weight or less is to attain a good balance between hardness and resilience, and elongation at break without ruining the straightness of the isocyanate. The polyol component is not limited to a specific one, and a polyether polyol or a polymer polyol is preferably used.
(5) A pad with an air bag, at least one spoke and a ring were integrally formed of a polyurethane, which was molded from a polyurethane material comprising an MDI prepolymer and a polyether polyol, said MDI prepolymer being denaturated with a long chain glycol having an average molecular weight of 2000 or more, and said polyether polyol containing one selected from a group consisting of 15 to 30 parts by weight of diethylene glycol, 11 to 20 parts by weight of ethylene glycol and 12 to 25 parts by weight of 1,4-butanediol for 100 parts by weight of polyether polyol. The reason why the above-described amount of diethylene glycol, ethylene glycol or 1,4-butanediol is added to the polyether polyol is that such an increased amount of these chain extenders more than the conventional and general amount (10 to 20 parts by weight for diethylene glycol, 5 to 10 parts by weight for ethylene glycol and 7 to 15 parts by weight for 1,4-butanediol, approximately) heightens the wear resistance. More preferably, 20 to 30 parts by weight of diethylene glycol or 15 to 20 parts by weight of 1,4-butanediol is added.
(6) A pad with an air bag, at least one spoke and a ring were integrally formed of a polyurethane, which was molded from a polyurethane material comprising an MDI prepolymer and a polyether polyol, said MDI prepolymer being a mixture of a prepolymer having two benzene nucleuses and a prepolymer having three benzene nucleuses with the ratio of the prepolymer having three benzene nucleuses to the total being 7% by weight or less, and said polyether polyol containing one selected from a group consisting of 15 to 30 parts by weight of diethylene glycol, 11 to 20 parts by weight of ethylene glycol and 12 to 25 parts by weight of 1,4-butanediol for 100 parts by weight of polyether polyol. See the measure (5) as to the added amount of the chain extenders.
Examples of a material used for molding the polyurethane in the measure (1) are polyurethane materials used in the measures (3) or (4). Examples of a material used for molding the polyurethane in the measure (2) are polyurethane materials used in the measures (5) or (6).
A mold release agent may be added to the polyether polyol or as a third component. It is also possible to add a coloring agent to the MDI prepolymer or the polyether polyol, or as a third component.
It is preferable that the polyurethane be an integral skin foam comprising an self-skin layer and a foamed core region. Examples of such a polyurethane are a polyurethane molded by injecting said polyurethane material into a mold cavity under reduced pressure to be foamed, or a polyurethane molded by injecting said polyurethane material and a foaming agent into a mold cavity under normal pressure to be foamed.
Further objects of this invention will become evident upon an understanding of the illustrative embodiments described below. Various advantages not specifically referred to herein but within the scope of the instant invention will occur to one skilled in the art upon practice of the presently disclosed invention. The following examples and embodiments are illustrative and not seen to limit the scope of the invention.